Conventional photosensitive materials for electrophotography using a flexible base such as a paper or a plastic film are comprised such that at least the layer contacting a photoconductive insulating layer has very low electrical resistance. Such a low electrical resistance layer is generally selected from a thin metal film (e.g. aluminum, copper, gold or silver), a carbon black containing layer or a low electrical resistance polymer layer. An aluminum foil is most conventionally used as a layer having low electrical resistance and high uniformity.
The photoconductive insulating layer is prepared by applying a composition which is prepared by dispersing photoconductive particles in lipophilic resins such as silicone resins, acrylate resins, polystyrenes, polyvinyl acetates, polyvinyl chlorides, polybutyl methacrylates, polyvinyl butyrals or copolymers thereof, onto a substrate and drying it.
The photoconductive particles are the particles of photoconductive substances such as zinc oxide, cadmium sulfide, titanium dioxide, etc.
The ratio of the photoconductive particles to binder resin in the photosensitive layer is from 3:1 to 20:1 and the dispersion is applied on the substrate in an amount such that the resulting photosensitive layer has a dry thickness of 5 to 30 microns.
Printing with a lithographic printing plate by electrophotography is performed as follows. An image to be printed is formed on the photoconductive layer by either dry or wet development according to conventional electrophotographic methods. Then, the photoconductive layer on which the image has been formed is treated with a desensitizing (etching) solution to make the nonimage area hydrophilic. In this case, the image area maintains ink sensitive. The thus treated lithographic printing plate is mounted on an offset printing machine and, as result, an oily ink is transferred to only the image area to provide a number of printed copy.
Thus, the lithographic printing plate is treated with a desensitizing (etching) solution and is immersed in damping water during printing. Therefore, the plate must have water resistance in order to provide a number of clean printed copies. Furthermore, it must have a smooth surface and sufficient adhesiveness in order to present peeling between layers constituting the plate. The plate is also required to retain its initial electrostatic characteristics and have good stability during storage.
If the damping water permeates into the paper base through the photosensitive layer during printing, the paper on the printing machine swells and sags to the extent such that the plate bends or breaks. Accordingly, sufficient printing durability cannot be maintained. In order to increase printing durability it is necessary to prevent the permeation of damping water into the paper through the photosensitive layer during printing. Therefore, an aluminum foil having a thickness of 0.1 micron or more is desirably formed between the photosensitive layer and the paper base. The aluminum foil does not have substantial mechanical strength, but serves as a barrier to prevent permeation of damping water through the photosensitive layer and can increase the electroconductivity of the base. Therefore, the aluminum foil is believed to improve the image quality and also the printing durability of the plate.
However, the photosensitive material composed of a photosensitive layer, aluminum foil and paper base has poor adhesiveness between the photosensitive layer and aluminum foil. Therefore, when such photosensitive material is used as a photosensitive material for lithographic printing, peeling occurs between the photosensitive layer and aluminum foil after the printing of a few copies and a number of clear printed copies cannot be produced.